Breastfeeding should break down mothers’ bones — here’s why it doesn’t

Benjamin Thompson

Welcome back to the Nature Podcast, this week: a new way for editing genes in gut bacteria...

Nick Petrić Howe

…and a hormone that builds bone during breastfeeding. I’m Nick Petrić Howe.

Benjamin Thompson

And I’m Benjamin Thompson.

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As we’ve covered on the podcast before, the bacteria that live in the human gut have been shown to play an important role in how our bodies operate and disrupting them can have serious health implications. But the gut is absolutely teeming with bacteria — different strains, different species — so working out which one or ones are responsible for various outcomes has been really difficult to tease out. But that might be about to change. This week a team have a paper out demonstrating a method to directly edit the genomes of specific bacteria in the guts of mice. Gemma Conroy, Senior Reporter for the Asia-Pacific region has been covering it for Nature, and she joins me on the line from Sydney. Gemma, thanks for being on the show.

Gemma Conroy

Thanks so much for having me.

Benjamin Thompson

Let’s start by talking about why it’s been so hard to home in and make changes to specific bacteria in the gut, because gene-editing tools like CRISPR- Cas, which is itself microbially derived of course, have been used in the lab for a good long while. So why has this been such a difficult thing to achieve?

Gemma Conroy

So it's not so much editing the bacteria, it's being able to access bacteria in the gut. The gut is such a complex environment, as you've mentioned, teeming with trillions of microbes, and it's just been really difficult to sort of find a tool, or develop a tool that can actually directly access these microbes in the gut. We've had some systems, you know, like CRISPR-Cas systems have been used to selectively kill bacteria carrying harmful genes, but this can increase the risk of microbes developing resistance. And then you have base editors which have, like, classically been delivered inside bacteriophages, which are viruses that infect bacteria. And base editing, by the way, is an editing tool that swaps one nucleotide base to another. So it's like converting an A to a T, for example. But these approaches have often failed to modify enough of the target bacteria population, so it hasn't been super effective. It's just it's really difficult to do, it's a very hard environment to access.

Benjamin Thompson

So, then it's not necessarily editing the genes that's been the difficult bit, it's been actually delivering the gene-editing machinery, whatever it is, to the correct bacteria in the gut. And that's what this paper, then, has overcome. And you mentioned bacteriophage there, these little viruses, and it seems like they've taken some of the abilities of these phages to deliver a gene-editing tool.

Gemma Conroy

Yeah, they engineered a vector out of components of bacteriophage. So it wasn't, you know, a natural bacteriophage. They sort of took elements of one to hone in specifically on E. coli, and this vector carried a base editor that targeted specific genes in E. coli. And they also managed to refine the system, because another concern with any kind of editing is that the edited genes will end up replicating and spreading to places where they shouldn't be. But they managed to engineer this base editor to sort of prevent those edited genes from replicating.

Benjamin Thompson

Okay, so they have this phage-derived system then that really can home in and deliver this base editor into this population of E. coli in the mouse gut. How effective was it? Like, did it work as intended?

Gemma Conroy

So, when they delivered the system to mice and they tweaked the system so it would target an E. coli gene that produces beta-lactamases, which are enzymes that drive bacterial resistance to several kinds of antibiotics. Some eight hours after mice received that treatment, I think it was around 93% the targeted bacteria had been edited.

Benjamin Thompson

So, 93% on the face of it is quite a high number then. How does this compare to previous efforts to edit genes inside an animal's gut?

Gemma Conroy

I think this is like the first time it's worked as well as it has. They were able to edit almost all of the colony in the gut.

Benjamin Thompson

So they tested it in mice on this one strain of E. coli then. But I guess, of course, the bacterial kingdom, as we know, is enormous and there's a bunch of different sorts of bacteria. Is this system applicable to others? Did they show anything else in that respect?

Gemma Conroy

So, in the lab, the researchers also used the tool to edit different strains of E. coli and they also tried it out on another species of bacteria that causes pneumonia infections, and they were able to still effectively edit these strains and this different species, which suggests that this system can be adapted. And you know, back to mice, they also use the base editor to edit the E. coli gene that produces this protein that drives bacterial colonization in the gut, and it's also thought to play a role in several neurodegenerative and autoimmune diseases. And they found that when they targeted this particular gene, the proportion of edited bacteria in mice hovered around 70% even three weeks after they were treated.

Benjamin Thompson

And as we said, this in general has been quite a difficult thing to do. What are folks saying about this work?

Gemma Conroy

Well, one researcher I spoke to said that this base editing system represents a critical leap forward in terms of developing editing tools that can modify bacteria directly inside the gut. That seems to be a key point of difference here. We're not just dealing with bacteria in a dish in a lab, but in a really complex environment, and that opens the possibility of editing microbes to combat disease and ensuring as well, that the modified DNA doesn't spread and lead to detrimental effects.

Benjamin Thompson

But of course, it's worth double underlining, we so often do, this work was done in mice or in a dish in the lab. Do the researchers think it will be applicable to humans? What are the next steps to try and find out if it is?

Gemma Conroy

I think a key thing is designing vectors that can be used in different models and different species, including humans and that will kind of be the next step. And it's also about tweaking the system as well so it can target different bacterial species. As you said before, it's not just E. coli. So, I think that would be a big next step.

Benjamin Thompson

Nature's Gemma Conroy, there. To read her news article on the new paper and to read the paper itself, look out for links in the show notes.

Nick Petrić Howe

Coming up, how are bones maintained during breastfeeding, a time when demands for calcium are particularly high? More on that later. Right now, though, it's time for the Research Highlights and we have a new voice on the podcast, Emily Bates.

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Emily Bates

It turns out, performing life-saving surgery is not something only humans can do, as some ants have been amputating their nest-mates’ legs to save lives. To reduce the risk of infection, some ant species make antimicrobial compounds that they apply to nest-mates’ wounds. But other species, including the Florida carpenter ant, have lost this ability. To understand how these insects handle wounds, researchers injured the legs of worker ants and studied their nest-mates’ reaction. When the injury was at the end of the leg, the insects focused on caring for the wound, but when an ant's leg was injured close to the body, nest-mates amputated the leg by biting it off, suggesting these ants can select their treatment method based on where the wound is located. Sink your teeth into that paper in Current Biology.

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For much of human history, amber was a prized commodity associated with power and wealth. So much so that in the Neolithic period, people would make fakes. Previous work has shown that amber was scarce in Spain during this period, and researchers have found a handful of lookalikes among genuine amber beads in the region. So, scientists have now analysed the simulated amber beads and found that most consisted of mollusc shells that had been coated in pine resin mixed with beeswax and a natural orange-red pigment, giving the items a smooth, amber-hued finish and making them a substitute for the real thing. You can read the full study in the Journal of Archaeological Science.

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Nick Petrić Howe

There’s a bit of a mystery when it comes to bones, how are they maintained while breastfeeding? You see, ordinarily the hormone oestrogen helps keep bones healthy, by maintaining bone formation and preventing them from being broken down. But oestrogen levels drop precipitously during lactation, a time when demands on the bone are incredibly high. To get enough calcium to make milk during breastfeeding, bones are stripped of the mineral. And while bone loss does occur during this time, mostly people are able to breastfeed without their bones completely eroding away. But without oestrogen how is this accomplished? Well, there are some clues in previous studies. A few years ago, a team found that deleting a certain gene in female mice led to them having unusually strong bones, but exactly how this happened wasn’t exactly clear. Now, though, the same team has gotten to the bottom of it, and discovered a protein that gets produced during lactation that is central to building bones. To find out more about this protein, I called up one of the authors, Holly Ingraham. She told me that they were pretty sure it was circulating in the blood, but finding out what exactly it was, was a whole other challenge.

Holly Ingraham

Well, so after you know, various things you know, trying to set up a standard biochemical assay which did not work, trying to do mass spec, which did not work. I forget what else we tried. We tried–we tried, almost everything we could think of. But it was actually when Muriel Babey, who's one of the lead along with William Kraus, she came to lab, she decided to put these mutant mice on a high-fat diet. I mean, we knew that a high-fat diet may change the activity of these neurons, but we had no idea that what it was going to do. Which is that when we put these mice on high fat diet, we lost the bone phenotype, and then we basically had a hook to figure out, okay, what are the gene changes that occur in the body of these mutant mice? And we found a set of genes were changed, and we narrowed it down to two. And then it turned out that the gene that was responsible or encoded the protein that was responsible for this phenotype was this protein called cellular communication network-3, CCN-3. And then we showed, in many, many ways, that this is the protein that is really having a huge effect on bone.

Nick Petrić Howe

And could you describe for me a couple of the ways that you showed this? Did you reduce the amount of this that the mice had? Increase it? That sort of thing, to sort of show the effects.

Holly Ingraham

Right. So, we did sort of that classic, let's get rid of it in this mutant mice, which we did, and we saw that the bone was lost. Or let's take wild type control mice, and let's over express this. And then when we did that, we found that bone formation, bone volume, bone strength, all increased. So we– we came at this multiple ways. And then we also worked with my co-lead, which is Tom Ambrosi he did an experiment with his collaborators where they took two-year-old male mice. So, these are pretty feeble ‘old gentlemen’, as we would call them, and they do a fracture on these femurs. And then what he did is he took CCN-3 in a slow-release gel, so it just releases slowly over time, and then he looks at fracture repair. So what was amazing is that the fracture repair was accelerated and improved in these two-year-old mice. And really, when you look at it, they look like a fracture repair of a two-month-old mouse. So, you're really rejuvenating this whole system. So when I saw that data, I said, this is real. I believe it.

Nick Petrić Howe

It's interesting though as well, because, like you say, this was in male mice, and you won't have thought that this hormone, this protein, would be something that they would really, you know, be dealing with.

Holly Ingraham

Well, okay, so that's the other beautiful thing about this project is that this is a female specific hormone, and we'll talk about when it comes on in normal females during lactation but what is really beautiful about this is it works on both male and female bone. So, the female specificity is just where it's being made and when it's being made by the female brain. But it's an equal opportunity hormone, so it works on both males and females, which is fantastic.

Nick Petrić Howe

So yeah, maybe that– we can talk about that, though. So what do you think this hormone is doing? Like, how does it play out in normal lactation cycle?

Holly Ingraham

Right. So, after identifying CCN-3 as the hormone that was responsible for the this bone phenotype in these mutant mice, we then asked, well, is this relevant? Is this relevant to any physiology? Normally in phenomena. So, we– we looked at every stage possible, and again, thinking, oh, well, maybe it comes on after weaning, because we know that bone builds up after weaning, and we were really surprised that this hormone comes on right during lactation. So, a little bit right after birth, this hormone appears in the brain. It comes on like gangbusters. And so then, you know, you go back to the literature. This is where I tell young people, read, read the old literature. You're not the first to have discovered something. And so that's what we ended up doing. And we found some amazing literature from 20 years ago and even longer that, it's been known that during lactation bone formation is up but the net result is bone loss because you're resorbing it to strip calcium off.

Nick Petrić Howe

Right? So, during lactation, bones are being stripped for calcium and without oestrogen you'd expect them to be completely broken down, but this hormone is kind of stepping up to the plate and preventing this from happening.

Holly Ingraham

Exactly. And so you need a way of ensuring that you don't just crumble into nothingness when you're breastfeeding.

Nick Petrić Howe

And I guess the other thing I was wondering as well is, your study was obviously focused on these mouse models. You have done a bit of work with human cells as well, but do you think the same thing happens in humans?

Holly Ingraham

Well, that's a wonderful question. And Muriel Babey, who is an adult endocrinologist, I think, is going to be working with some clinical people, once we can get samples. And– and this stuff is really hard to detect in the blood because there's not really good reagents right now, which is the other thing we're trying to do. But I think once we get those reagents, that's one of the first questions. Is this relevant to humans? I'm going to guess, yes, but that has yet to be proved.

Nick Petrić Howe

So you've got an idea of what role this hormone plays during breastfeeding, but I wonder what you think are the broader implications of this finding. There are a lot of diseases that affect bones, things like osteoporosis, where bones become weaker. Could this hormone be potentially something that people could look to.

Holly Ingraham

Right, so I think that in terms of the translational aspects of this, I do think hopefully we'll get there someday. We hope that we can, because this is a natural hormone, very much like a GLP one that has now been used for obesity. And, you know, I'm hoping with new technology, we could really see if this can be translated. And I do think that the fracture repair data suggests that it looks promising. And I also think that it– there was just an article in the Washington Post describing a woman that had lactational osteoporosis, which can be really severe. And so, one wonders whether part of this whole pathway is disturbed in women like that, so that they're fine until they get challenged with this pregnancy and lactation. And so, the question is, are we going to find variants of this pathway in women like that that are susceptible to lactational osteoporosis?

Nick Petrić Howe

That was Holly Ingraham, of the University of California, San Francisco, in the US. For more on that story, check out the show notes for some links.

Benjamin Thompson

Finally, on the show, it's time for the Briefing Chat where we talk about a couple of stories that have been highlighted in the Nature Briefing. Nick, why don't you go first this week, what have you got?

Nick Petrić Howe

So this week, I've been reading a story that's reported on in Nature about a science paper looking at the sort of root cause of migraines.

Benjamin Thompson

Right, now, I must say, I've never had a migraine, but I know a lot of people who have, and they sound absolutely awful and can be debilitating in many cases. And I think, yeah, bit of a mystery as to what's going on, right?

Nick Petrić Howe

Yeah, it's a bit of a mystery. Because one of the things that's really unclear is how things happening in the brain lead to pain, because the brain, as you know, doesn't actually have any pain receptors. That's why, if you've ever seen like a surgery on someone's brain, they can do it without putting them under anaesthesia. So what's going on within the brain to cause pain? Because it's not the brain itself. And so this study, which is another study in mice, it seems to be a bit of a theme of the show this week, they've looked at a particular type of migraine called an aura migraine. And I don't know if you've familiar with this, but an aura migraine is where you have an aura preceding the migraine. So this is where you have sensitivity to light. You have, like, dark spots and other stuff going on. But the key thing for this is you have sort of a blackout during it, so neuronal activity shuts off for a short amount of time, and then the migraine comes. That's the one they've been investigating here.

Benjamin Thompson

Right? So there's kind of a tell that it's about to start, and then the pain kicks in, right? So tell me how and some unusual stuff going on in the brain of these mice, then what do the researchers find specifically?

Nick Petrić Howe

So what they found was happening is when this blackout occurred, there were changes to the cerebral spinal fluid. So this is the fluid that sort of surrounds the spine and the brain. And what seems to be happening here is this neuronal blackout seems to proceed like changes to the cerebral spinal fluid, so changes in protein specifically. So some proteins really jump up in the number that are present, and some proteins really drop down. And one in particular, that jumps up is one called CGRP, which is known to transmit pain and has previously been a target of sort of migraine therapies. So a lot changing in the cerebral spinal fluid.

Benjamin Thompson

Right. And presumably, then this has knock on effects on pain neurons or elsewhere in the body.

Nick Petrić Howe

Well, what is unusual here is they've actually uncovered, like a previously unknown part of the anatomy. So there is actually a little gap, and it's at this area known as the trigeminal ganglion, which is like a bunch of nerves at the base of the skull where the cerebral spinal fluid can sort of come in and may be having its effects. And this trigeminal ganglion, this is a centre for sort of transmitting pain and other sensory information. So the researchers believe what is happening here is some of these proteins, these changes in the proteins are causing an effect on this area, and then that is leading to pain.

Benjamin Thompson

And so it seems like in mice, at least, as you say, this is giving an insight into, maybe the biochemistry, I suppose, the molecular biology underlining this particular type of migraine. Any idea of whether this might translate into humans? I guess there are people who will be crying out for treatments for migraine.

Nick Petrić Howe

Yeah well, that remains to be seen. So the key thing will be trying to identify what proteins may be changing here, and then seeing what effects they have. And there'll obviously be future studies that would involve humans to sort of figure this out and maybe see if certain proteins could be blocked or that sort of thing to help it. But the interesting thing, according to some of the researchers who interviewed for this article, is that this shows that a change in the brain is then preceding pain, which could be like a link between the two, which has previously been unknown. And the other big question for researchers as well is, why is this causing a headache? As I said, this trigeminal ganglion, it's like a core centre for sensory information. So why are we just getting a headache? Why not pain everywhere and that sort of thing? So a lot more questions for this one, but certainly answers, at least some, that have long puzzled scientists.

Benjamin Thompson

Well, definitely one to follow and fingers crossed then that something useful can be found that can translate to help people in the future. But let's move on to my story this week, Nick and it couldn't be more different, and it's about fashion. Now you and I are both incredibly well-dressed podcast hosts. Listeners can't see it, but here we are in our immaculately-cut Italian suits. Not sure about your top hat, but let's leave that be. And I know that fashion is central to our brand, right? I know something else that's central to our brand, and that's recycling, right? And I've got a story here that combines both of them.

Nick Petrić Howe

Right, okay, so I've heard the term fast-fashion to describe the sort of waste associated with fashion. I'm assuming, if this is about recycling, this is something to make something to maybe help the sort of waste that accumulates with the industry of fashion.

Benjamin Thompson

Yeah, well, absolutely right. Estimates suggest that less than 1% of clothes are recycled, and nearly three quarters of garments either end up just incinerated or in landfill. And that can have knock on effects for things like microplastics, from synthetic fibres getting into waterways and all the other sort of stuff that we've covered on the podcast. But researchers are trying to work out if they can improve recycling. And so this is a story I read about in Nature, and it's based on a paper in Scientific Advances. Now, one of the issues with recycling clothes is the fact they're often a blend of materials, cotton and polyester, say, now if you think about recycling, you often have to separate out the individual things, right? Maybe at home, you separate out your cans from your glass, that sort of thing. But when the materials are actually kind of woven or stitched together, that can be really, really difficult and mechanical recycling techniques struggle to separate multi-fibre textiles. And so, researchers have turned to chemistry to try and crack this nut.

Nick Petrić Howe

Okay, so I think you know, we've covered on the podcast before, maybe sort of like enzymes and that sort of thing to break down plastics. Is this something like that?

Benjamin Thompson

Slightly different. So it doesn't use enzymes. It does use a catalyst and heat and a technique called microwave assisted glycolysis. And what this does is it breaks down large-chain polymers into smaller units, right, which is kind of what we want, and those smaller units can be re-used. Now in this paper, the team tested it on fabrics that were either 100% polyester or a 50/50, poly-cotton. And what they showed is, with pure polyester, the reaction converted over 90% of the material into a molecule called BHET, which can be reused to make more polyester. But what's neat is, for the mix, the process doesn't touch the cotton right, so the polyester is broken down and leaving the cotton behind, and the cotton can be recycled in its own way that's often shredded up and used to make new fibres. So you've got this thing that is quite inherently linked, but the team have managed to unstitch it, if you will, to try and put individual materials down different recycling paths.

Nick Petrić Howe

And did they try this on other fabrics and things? Because I guess there's more than polyester and cotton that make up many clothes.

Benjamin Thompson

Yeah, absolutely right. And they tried it with different combinations. Okay, so the results seem positive when they just tried essentially like a random group of garments, right that contained unknown proportions of fibres, including cotton, polyester, nylon or elastane, which is sometimes known as spandex, okay. And they used their system, and the spandex broke down into a useful molecule called MDA, and the polyester broke down as before. So they showed that they could, once again, sort of split these things off. And what's neat about this is that it's quick, okay? The team optimized this, so in some cases, it only takes 15 minutes to do the job, right–

Nick Petrić Howe

–oh, wow–

Benjamin Thompson

-so quite cost effective, because one of the researchers involved in this work said that typically, it can take days to break down some of these synthetic materials, and he says that eventually, potentially, they can get it down to seconds.

Nick Petrić Howe

Whoa. Okay, that does sound very quick, but I guess, with my sort of sensible top hat on, how are they going to translate this into something we can use in the real world? I'm assuming this is just sort of testing at the moment.

Benjamin Thompson

I mean, yes, that's always the case. What's interesting, though, is that they've done some analysis, and they calculate that maybe this process could allow 88% of clothing worldwide to be recycled. So you need less time to sort it, it's a relatively simple process. I think the researchers themselves say if it could be taken up in the real world. But of course, there are some caveats. Some polyesters don't break down so well, maybe if they've been dyed or if they've been treated to be resistant to UV or fire. So there's a little ways to go, and it is a proof of concept. But what goes without saying, and what one researcher quoted in the article does say, is that “repairing and reusing” really should be sort of the key weapons in the fight against this textile waste. But this could be a useful addition.

Nick Petrić Howe

Well, this does sound like it could be incredibly useful if perfected. And you know, some of these caveats are overcome. But for now, I think that's all we've got time for on the Briefing Chat. And listeners for more on those stories and for where you can sign up to Nature Briefing to get more like them. Check out the show notes for some links.

Benjamin Thompson

As always, you can keep in touch with us on X we're @NaturePodcast, or you can send us an email to podcast@nature.com I'm Benjamin Thompson.

Nick Petrić Howe

And I'm Nick Petrić Howe. Thanks for listening.